1. Field of the Invention
The invention relates to a scanning device comprising a turntable for supporting a disc-shaped recording medium, which turntable is rotatable about an axis of rotation, and further comprising a radiation source and a positioning device which is operable with respect to the turntable in a plane which is oriented at least substantially transversely to the axis of rotation, which positioning device comprises an actuator unit, which comprises a base and a carrier which is movable relative to the base and which carries an objective system, which positioning device further comprises a directing unit for directing a radiation beam emitted by the radiation source to the objective system.
2. Description of Related Art
Such a scanning device is known from EP-A 0 464 912. The known scanning device has a turntable for supporting, centering and rotating optical discs, particularly Compact Discs. Such discs have an information layer with a spiral track formed with a succession of pits, the information being stored in the track in digital form and the pits representing series of bits. The digital information on a Compact disc is covered by a transparent substrate having a nominal thickness of 1.2 mm. The scanning device further comprises an electro-optical scanning unit comprising an objective lens and an actuator for moving the lens. For optically scanning a disc a semiconductor-laser source is used, the beam of laser light being focused onto the information layer of the Compact Disc via the objective lens, the substrate being disposed between the objective lens and the information layer. Since the digital information on a Compact Disc is protected by a substrate possible flaws and dust particles are not situated in the focal plane of the light beam which scans the disc, so that they have comparatively little influence.
In order to set a correct focusing of the light beam the actuator comprises a stationary actuator section and a movable actuator section and comprises a drive combination of a coil and a permanent magnet for moving the objective lens parallel to its optical axis. During focusing the movable actuator section also performs translational movements about a central position. A focus servosystem provides the coil drive. The actuator is also capable of moving the objective lens in a direction transverse to its optical axis for the purpose of tracking.
In the course of time optical discs of various types, based on the "Compact Disc Standard", have been developed, such as CD-Audio, CD-ROM, CD-I and CD-R, which all have a substrate whose thickness complies with the value prescribed by this standard. Recently, a digital versatile disc (DVD) has been proposed, which comprises two transparent substrates between which one or more information layers are interposed. The substrates of said versatile disc each have a nominal thickness of 0.6 mm, i.e. half the substrate thickness of the Compact Discs. Furthermore, the bit length and the track pitch in the DVD differ from the corresponding parameters of the Compact Disc. Owing to these and still other differences the digital versatile disc, which is presented in various versions such as DVD-Audio, DVD-ROM, DVD-RAM and DVD-Video, cannot readily be scanned by means of the known scanning device.